Finely ground gypsum has many uses. It is added to cosmetics, paper or coatings as a filler. Pure, fine ground gypsum is also useful as a calcium supplement in foods or pharmaceuticals. Preparation of the ground gypsum is done using any known grinding method, particularly wet grinding. The finely ground solid product is useful as a filler in coatings, paper or cosmetics. Ground gypsum is also useful as an additive to food or pharmaceuticals.
Wet grinding of gypsum is an economical method of producing a powder having average particle sizes less than 10 microns. A slurry of gypsum in water flows through a comminution or grinding device, such as a ball mill, and can be recycled through the comminution device as much as necessary to obtain the desired particle size range. It is desirable to grind a slurry that is highly loaded for use in paper coatings. However, highly loaded slurries are difficult to pump. For example, a slurry having a 70% loading by weight (70 wt % gypsum and 30 wt % water), for all useful purposes does not flow at all.
Even when a pumpable slurry is attained, it is difficult to keep it flowing. It has been found that as grinding progresses, the slurry thickens and becomes viscous as the average particle size decreases. While not wishing to be bound by theory, it is believed that grinding exposes fresh, reactive gypsum faces that tightly bind water molecules, causing the slurry to thicken. Thickening of the slurry requires additional energy to pump it through the grinding device, increasing the cost of the entire process.
Dispersants are known for reducing the viscosity of a slurry or solution, but the response of particular dispersants to a specific slurry or solution is unpredictable. For example, dispersants known for use with cements are not necessarily effective in dispersing gypsum solutions. Of the dispersants known to be useful for gypsum slurries, some cause severe retardation in the setting of the hemihydrate form. Knowledge that a particular dispersant is effective in dispersing calcium sulfate slurries does not ensure that the dispersant is suitable for a particular application.
Processes are also known for milling gypsum and a titanium dioxide pigment together in the presence of a dispersant, as shown in U.S. Pat. No. 5,338,354. However, this patent fails to teach or suggest the problem identified and solved by Applicants, the problem of thickening of the gypsum slurry during grinding. There is no suggestion that polycarboxylate dispersants are useful for wet grinding of gypsum in the absence of titanium oxide or that natural landplaster or terra alba are suitable feed materials.
Cement additive compositions that include polycarboxylate dispersants are disclosed in U.S. Pat. No. 6,800,129 (“'129 Patent”). Combining at least two stable suspensions makes the additive composition. The additive composition may be used with clinker or gypsum before or during the grinding operation. However, this reference fails to disclose wet grinding of the feed material, and it would not be understood by an artisan that wet grinding was contemplated. Addition of water to clinker would cause the cement to start to set. The teachings of the '129 Patent would not be suitable for finely grinding the gypsum slurry of this invention.